3.1 Life’s molecular diversity is based on the properties of carbon Diverse molecules found in cells are composed of carbon bonded to other elements

3.1 Life’s molecular diversity is based on the properties of carbon

Diverse molecules found in cells are composed of carbon bonded to other elements

– Carbon-based molecules are called organic compounds

– By sharing electrons, carbon can bond to four other atoms

– By doing so, it can branch in up to four directions

Copyright © 2009 Pearson Education, Inc.


Methane (CH4) is one of the simplest organic compounds

– Four covalent bonds link four hydrogen atoms to the carbon atom

– Each of the four lines in the formula for methane represents a pair of shared electrons


Structuralformula

Ball-and-stickmodel

Space-fillingmodel

Methane

The four single bonds of carbon point to the cornersof a tetrahedron.


Methane and other compounds composed of only carbon and hydrogen are called hydrocarbons

– Carbon, with attached hydrogens, can bond together in chains of various lengths


A chain of carbon atoms is called a carbon skeleton

– Carbon skeletons can be branched or unbranched

– Therefore, different compounds with the same molecular formula can be produced

– These structures are called isomers



3.2 Characteristic chemical groups help determine the properties of organic compounds

An organic compound has unique properties that depend upon

– The size and shape of the molecule and

– The groups of atoms (functional groups) attached to it

A functional group affects a biological molecule’s function in a characteristic way



Compounds containing functional groups are hydrophilic (water-loving)

– This means that they are soluble in water, which is a necessary prerequisite for their roles in water-based life



The functional groups are

– Hydroxyl group—consists of a hydrogen bonded to an oxygen

– Carbonyl group—a carbon linked by a double bond to an oxygen atom

– Carboxyl group—consists of a carbon double-bonded to both an oxygen and a hydroxyl group

– Amino group—composed of a nitrogen bonded to two hydrogen atoms and the carbon skeleton

– Phosphate group—consists of a phosphorus atom bonded to four oxygen atoms


3.3 Cells make a huge number of large molecules from a small set of small molecules

There are four classes of biological molecules

– Carbohydrates

– Proteins

– Lipids

– Nucleic acids



The four classes of biological molecules contain very large molecules

– They are often called macromolecules because of their large size

– They are also called polymers because they are made from identical building blocks strung together

– The building blocks are called monomers


A cell makes a large number of polymers from a small group of monomers

– Proteins are made from only 20 different amino acids, and DNA is built from just four kinds of nucleotides

The monomers used to make polymers are universal




Monomers are linked together to form polymers through dehydration reactions, which remove water

Polymers are broken apart by hydrolysis, the addition of water

All biological reactions of this sort are mediated by enzymes, which speed up chemical reactions in cells


Animation: Polymers

Unlinkedmonomer

Short polymer

Unlinkedmonomer

Short polymer

Longer polymer

Dehydrationreaction

Hydrolysis

CARBOHYDRATES


3.4 Monosaccharides are the simplest carbohydrates

Carbohydrates range from small sugar molecules (monomers) to large polysaccharides

– Sugar monomers are monosaccharides, such as glucose and fructose

– These can be hooked together to form the polysaccharides


3.4 Monosaccharides are the simplest carbohydrates

The carbon skeletons of monosaccharides vary in length

– Glucose and fructose are six carbons long

– Others have three to seven carbon atoms

Monosaccharides are the main fuels for cellular work

– Monosaccharides are also used as raw materials to manufacture other organic molecules


Glucose(an aldose)

Fructose(a ketose)

3.5 Cells link two single sugars to form disaccharides

Two monosaccharides (monomers) can bond to form a disaccharide in a dehydration reaction

– An example is a glucose monomer bonding to a fructose monomer to form sucrose, a common disaccharide


Animation: Disaccharides

Glucose Glucose

Glucose Glucose

Maltose

3.7 Polysaccharides are long chains of sugar units

Starch is a storage polysaccharide composed of glucose monomers and found in plants

Glycogen is a storage polysaccharide composed of glucose, which is hydrolyzed by animals when glucose is needed

Cellulose is a polymer of glucose that forms plant cell walls

Chitin is a polysaccharide used by insects and crustaceans to build an exoskeleton


3.7 Polysaccharides are long chains of sugar units

Polysaccharides are hydrophilic (water-loving)

– Cotton fibers, such as those in bath towels, are water absorbent


Animation: Polysaccharides

LIPIDS


3.8 Fats are lipids that are mostly energy-storage molecules

Lipids are water insoluble (hydrophobic, or water fearing) compounds that are important in energy storage

– They contain twice as much energy as a polysaccharide

Fats are lipids made from glycerol and fatty acids



Fatty acids link to glycerol by a dehydration reaction

– A fat contains one glycerol linked to three fatty acids

– Fats are often called triglycerides because of their structure


Animation: Fats

Fatty acid

Glycerol


Some fatty acids contain double bonds

– This causes kinks or bends in the carbon chain because the maximum number of hydrogen atoms cannot bond to the carbons at the double bond

– These compounds are called unsaturated fats because they have fewer than the maximum number of hydrogens

– Fats with the maximum number of hydrogens are called saturated fats


3.9 Phospholipids and steroids are important lipids with a variety of functions

Phospholipids are structurally similar to fats and are an important component of all cells

– For example, they are a major part of cell membranes, in which they cluster into a bilayer of phospholipids

– The hydrophilic heads are in contact with the water of the environment and the internal part of the cell

– The hydrophobic tails band in the center of the bilayer


Water

Hydrophobictails

Hydrophilicheads

Water

PROTEINS


3.11 Proteins are essential to the structures and functions of life

A protein is a polymer built from various combinations of 20 amino acid monomers

– Proteins have unique structures that are directly related to their functions

– Enzymes, proteins that serve as metabolic catalysts, regulate the chemical reactions within cells


3.11 Proteins are essential to the structures and functions of life

Structural proteins provide associations between body parts and contractile proteins are found within muscle

Defensive proteins include antibodies of the immune system, and signal proteins are best exemplified by the hormones

Receptor proteins serve as antenna for outside signals, and transport proteins carry oxygen


3.12 Proteins are made from amino acids linked by peptide bonds

Amino acids, the building blocks of proteins, have an amino group and a carboxyl group

– Both of these are covalently bonded to a central carbon atom

– Also bonded to the central carbon is a hydrogen atom and some other chemical group symbolized by R


Carboxylgroup

Aminogroup


Amino acids are classified as hydrophobic or hydrophilic

– Some amino acids have a nonpolar R group and are hydrophobic

– Others have a polar R group and are hydrophilic, which means they easily dissolve in aqueous solutions


Leucine (Leu)

Hydrophobic

Serine (Ser)

Hydrophilic

Aspartic acid (Asp)


Amino acid monomers are linked together to form polymeric proteins

– This is accomplished by an enzyme-mediated dehydration reaction

– This links the carboxyl group of one amino acid to the amino group of the next amino acid

– The covalent linkage resulting is called a peptide bond


Carboxylgroup

Amino acid

Aminogroup

Amino acid

Carboxylgroup

Amino acid

Aminogroup

Amino acid

Peptidebond

Dipeptide

Dehydrationreaction

3.13 A protein’s specific shape determines its function

A polypeptide chain contains hundreds or thousands of amino acids linked by peptide bonds

– The amino acid sequence causes the polypeptide to assume a particular shape

– The shape of a protein determines its specific function


Groove

3.13 A protein’s specific shape determines its function

If for some reason a protein’s shape is altered, it can no longer function

– Denaturation will cause polypeptide chains to unravel and lose their shape and, thus, their function

– Proteins can be denatured by changes in salt concentration and pH


3.14 A protein’s shape depends on four levels of structure

A protein can have four levels of structure

– Primary structure

– Secondary structure

– Tertiary structure

– Quaternary structure


3.14 A protein’s shape depends on four levels of structure

The primary structure of a protein is its unique amino acid sequence

– The correct amino acid sequence is determined by the cell’s genetic information

– The slightest change in this sequence affects the protein’s ability to function


Four Levels of Protein Structure

Amino acids

Primary structure

Alpha helix

Hydrogenbond

Secondary structure

Pleated sheet

Polypeptide(single subunitof transthyretin)

Tertiary structure

Transthyretin, withfour identicalpolypeptide subunits

Quaternary structure

Amino acids

Primary structure

3.15 TALKING ABOUT SCIENCE: Linus Pauling contributed to our understanding of the chemistry of life

After winning a Nobel Prize in Chemistry, Pauling spent considerable time studying biological molecules

– He discovered an oxygen attachment to hemoglobin as well as the cause of sickle-cell disease

– Pauling also discovered the alpha helix and pleated sheet of proteins


Short polymer MonomerHydrolysis

Dehydration

Longer polymer

Sucrose

Temperature (°C)

0 20

Rat

e o

fre

acti

on

Enzyme A

100

Enzyme B

40 60 80

You should now be able to

1. Discuss the importance of carbon to life’s molecular diversity

2. Describe the chemical groups that are important to life

3. Explain how a cell can make a variety of large molecules from a small set of molecules

4. Define monosaccharides, disaccharides, and polysaccharides and explain their functions

5. Define lipids, phospholipids, and steroids and explain their functions


You should now be able to

6. Describe the chemical structure of proteins and their importance to cells

7. Describe the chemical structure of nucleic acids and how they relate to inheritance


Documents

3.1 Life’s molecular diversity is based on the properties of carbon Diverse molecules found in cells are composed of carbon bonded to other elements